Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, radios and other devices that receive RF signals. RF receivers typically require frequency translation or mixing. For example, with respect to FM audio broadcasts, FM radio receivers may translate one broadcast channel in the FM frequency band to an intermediate frequency. Within the United States, FM radios will typically translate FM audio signals, which are broadcast in 200 KHz channels in the frequency band from 88 MHz to 108 MHz, to an intermediate frequency of 10.7 MHz. FM demodulators and stereo decoders can then convert this 10.7 MHz IF signal to demodulated left and right audio signal that can be sent to stereo speakers. Although other countries will have different frequency bands and channel spacing, the reception of audio broadcast signals, such as FM audio broadcasts, is similarly accomplished using RF receivers.
The majority of typical RF receivers perform frequency translation or mixing using an oscillator and an analog multiplier or mixer. An oscillator will typically output a local oscillator (LO) signal in the form of a sine wave or periodic wave having a tuned frequency (fLO). A mixer then mixes the RF input signal spectrum, which includes desired spectral content at a target channel having a particular center frequency (fCH), with the LO signal to form an output signal having spectral content at frequencies equal to the sum and difference of the two input frequencies, namely fCH+fLO and fCH−fLO. One of these components forms the channel center frequency translated to the desired IF frequency, and the other component can be filtered out. The oscillator can be implemented with a variety of circuits, including, for example, a tuned inductor-capacitor (LC) oscillator, a charge relaxation oscillator, or a ring oscillator.
Many receiver systems operate by implementing either high-side injection or low-side injection of the LO mixing signal into the mixer to mix the input signal spectrum to the desired IF frequency. In particular, a mixer in a standard receiver system will mix a channel within a signal spectrum to a desired IF frequency in accordance with the equation fCH−fLO=fIF, if low-side injection is utilized, and in accordance with the equation fLO−fCH=fIF, if high-side injection is utilized. Most receiver systems do not provide the ability to switch between high-side injection and low-side injection during operation. Some prior systems, however, have attempted to select between high-side and low-side injection during operation by assessing the level of noise or spurs in the tuned channel once it has been received and processed. This solution, however, looks at the result of the LO injection selection and does not take into consideration potential images that could be caused by signal power within the signal spectrum not located at the frequency of the channel to be tuned.